Tuesday, September 22, 2015

Summary: Using the technique from the previous post, here are three space leaks I found.

Every large Haskell program almost inevitably contains space leaks. This post examines three space leaks I found while experimenting with a space-leak detection algorithm. The first two space leaks have obvious causes, but I remain mystified by the third.

Hoogle leak 1

The motivation for looking at space leak detection tools was that Hoogle 5 kept suffering space leaks. Since Hoogle 5 is run on a virtual machine with only 1Gb of RAM, a space leak will often cause it to use the swap file for the heap, and destroy performance. I applied the detection techniques to the hoogle generate command (which generates the databases), which told me that writeDuplicates took over 100K of stack. The body of writeDuplicates is:

(Debugging tip: always use print for debugging and never for real code, that way getting rid of all debugging output is easy.) It failed after printing step 2, but before printing step 3. Pulling each subexpression out and repeating the evaluate/rnf pattern I reduced the expression to:

Map.fromListWith (\(x1,x2) (y1,y2) -> (min x1 y1, x2 ++ y2)) xs

The fromListWith function essentially performs a foldl over values with duplicate keys. I was using Data.Map.Strict, meaning it the fold was strict, like foldl'. However, the result is a pair, so forcing the accumulator only forces the pair itself, not the first component, which contains a space leak. I effectively build up min x1 (min x1 (min x1 ... in the heap, which would run faster and take less memory if reduced eagerly. I solved the problem with:

Map.fromListWith (\(x1,x2) (y1,y2) -> (, x2 ++ y2) $! min x1 y1) xs

After that the stack limit could be reduced a bit. Originally fixed in commit 102966ec, then refined in 940412cf.

I had already added ! in the definition of f when writing it, on the grounds it was likely a candidate for space leaks (an accumulating map), so was immediately suspicious that I hadn't got it right. However, adding bang patterns near real made no difference, so I tried systematically reducing the bug.

Since this code isn't in IO, the evaluate technique from the previous leak doesn't work. Fortunately, using seq works, but is a bit more fiddly. To check the argument expression (reverse . sortOn) wasn't leaking I made the change:

I was slightly worried that the GHC optimiser may break the delicate seq/trace due to imprecise exceptions, but at -O0 that didn't happen. Successive attempts at testing different subexpressions eventually lead to genericLength xs, which in this case returns a Word16. The definition of genericLength reads:

genericLength [] = 0
genericLength (_:l) = 1 + genericLength l

Alas, a very obvious space leak. In addition, the base library provides two rules:

If you use genericLength on Int or Integer then it is replaced with a strict version without a space leak - but on Word16 the space leak remains. To solve this space leak I replaced genericLength xs with fromIntegral (length xs) in commit 12c46e93, which worked. After that change, the Hoogle test suite can be run with 1Kb of stack - a test that has been added to the continuous integration.

Shake leak

After solving the space leak from the original post, I was then able to run the entire test suite with 1Kb stack on my Windows machine. I made that an RTS option to the Cabal test suite, and my Linux continuous integration started failing. Further experimentation on a Linux VM showed that:

The entire test failed at 50K, but succeeded at 100K.

The excessive stack usage could be replicated with only two of the tests - the tar test followed by the benchmark test. The tar test is incredibly simple and likely any of the tests before before benchmark would have triggered the issue.

The tests succeeded in 1K if running benchmark followed by tar.

The initial assumption was that some CAF was being partially evaluated or created by the first test, and then used by the second, but I have yet to find any evidence of that. Applying -xc suggested a handful of possible sites (as Shake catches and rethrows exceptions), but the one that eventually lead to a fix was extractFileTime, defined as:

There is a small (constant sized) space leak here - the result does not force extractTime, but returns a pair containing thunks. In fact, getFileStatus from the unix library allocates a ForeignPtr to store s, so by not forcing the pair we cause the ForeignPtr to live much longer than would be otherwise required. The fix from commit 2ee36a99 is simple:

Monday, September 21, 2015

Summary: Below is a technique for easily detecting space leaks. It's even found a space leak in the base library.

Every large Haskell program almost inevitably contains space leaks. Space leaks are often difficult to detect, but relatively easy to fix once detected (typically insert a !). Working with Tom Ellis, we found a fairly simple method to detect such leaks. These ideas have detected four space leaks so far, including one in the base library maximumBy function, which has now been fixed. For an introduction to space leaks, see this article.

Our approach is based around the observation that most space leaks result in an excess use of stack. If you look for the part of the program that results in the largest stack usage, that is the most likely space leak, and the one that should be investigated first.

Method

Given a program, and a representative run (e.g. the test suite, a suitable input file):

Run the program with a specific stack size, e.g. ./Main +RTS -K100K to run with a 100Kb stack.

Increase/decrease the stack size until you have determined the minimum stack for which the program succeeds, e.g. -K33K.

Reduce the stack by a small amount and rerun with -xc, e.g. ./Main +RTS -K32K -xc.

The -xc run will print out the stack trace on every exception, look for the one which says stack overflow (likely the last one) and look at the stack trace to determine roughly where the leak is.

Attempt to fix the space leak, confirm by rerunning with -K32K.

Repeat until the test works with a small stack, typically -K1K.

Add something to your test suite to ensure that if a space leak is ever introduced then it fails, e.g. ghc-options: -with-rtsopts=-K1K in Cabal.

I have followed these steps for Shake, Hoogle and HLint, all of which now contain -K1K in the test suite or test scripts.

Example: Testing on Shake

Applying these techniques to the Shake test suite, I used the run ./shake-test self test, which compiles Shake using Shake. Initially it failed at -K32K, and the stack trace produced by -xc was:

*** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace:
Development.Shake.Profile.generateSummary,
called from Development.Shake.Profile.writeProfile,
called from Development.Shake.Core.run.\.\.\,
called from Development.Shake.Core.run.\.\,
called from Development.Shake.Database.withDatabase.\,
called from Development.Shake.Storage.withStorage.continue.\,
called from Development.Shake.Storage.flushThread,
called from Development.Shake.Storage.withStorage.continue,
called from Development.Shake.Storage.withStorage.\,
called from General.FileLock.withLockFile.\,
called from General.FileLock.withLockFile,
called from Development.Shake.Storage.withStorage,
called from Development.Shake.Database.withDatabase,
called from Development.Shake.Core.run.\,
called from General.Cleanup.withCleanup,
called from Development.Shake.Core.lineBuffering,
called from Development.Shake.Core.run,
called from Development.Shake.Shake.shake,
called from Development.Shake.Args.shakeArgsWith,
called from Test.Type.shakeWithClean,
called from Test.Type.shaken.\,
called from Test.Type.noTest,
called from Test.Type.shaken,
called from Test.Self.main,
called from Test.main,
called from :Main.CAF:main
stack overflow

Looking at the generateSummary function, it takes complete profile information and reduces it to a handful of summary lines. As a typical example, one line of the output is generated with the code:

Most of the code is map, maximum and sum in various combinations. By commenting out pieces I was able to still produce the space leak using maximumBy alone. By reimplementing maximumBy in terms of foldl', the leak went away. Small benchmarks showed this space leak was a regression in GHC 7.10, which I reported as GHC ticket 10830. To fix Shake, I added the helper:

maximumBy' cmp = foldl1' $ \x y -> if cmp x y == GT then x else y

After switching to maximumBy' I was able to reduce the stack to -K1K. While this space leak was not problematic in practice (it's rarely used code which isn't performance sensitive), it's still nice to fix. I modified the Shake test suite to pass -K1K so if I ever regress I'll get an immediate notification. (Shake actually had one additional Linux-only space leak, also now fixed, but that's a tale for a future post.)

Caveats

This method has found several space leaks - two in Shake and two in Hoogle (I also ran it on HLint, which had no failures). However, there are a number of caveats:

GHC's strictness analyser often removes space leaks by making accumulators strict, so -O2 tends to remove some space leaks, and profiling may reinsert them by blocking optimisations. I currently check my code using -O0, but using libraries I depend on with whatever optimisation they install with by default. By ensuring optimisations do not remove space leaks, it is less likely that minor code tweaks will introduce space leaks due to missed optimisations.

The stack trace produced by -xc omits duplicate adjacent elements, which is often the interesting information when debugging a stack overflow. In practice, it's a little inconvenient, but not terrible. Having GHC provide repetition counts (e.g. Main.recurse *12) would be useful.

The stack traces don't contain entries for things in imported libraries, which is unfortunate, and often means the location of the error is a 20 line function instead of the exact subexpression. The lack of such information makes fixing leaks take a little longer.

The -xc flag prints stack information on all exceptions, which are often numerous. Lots of IO operations make use of exceptions even when they succeed. As a result, it's often easier to run without -xc to figure out the stack limit, then turn -xc on. Usually the stack overflow exception is near the end.

There are sometimes a handful of exceptions after the stack overflow, as various layers of the program catch and rethrow the exception. For programs that catch exceptions and rethrow them somewhat later (e.g. Shake), that can sometimes result in a large number of exceptions to wade through. It would be useful if GHC had an option to filter -xc to only certain types of exception.

Some functions in the base libraries are both reasonable to use and have linear stack usage - notably mapM. For the case of mapM in particular you may wish to switch to a constant stack version while investigating space leaks.

This technique catches a large class of space leaks, but certainly not all. As an example, given a Map Key LargeValue, if you remove a single Key but don't force the Map, it will leak a LargeValue. When the Map is forced it will take only a single stack entry, and thus not be detected as a leak. However, this technique would have detected a previous Shake space leak, as it involved repeated calls to delete.

Feedback

If anyone manages to find space leaks using this technique we would be keen to know. I have previously told people that there are many advantages to lazy programming languages, but that space leaks are the big disadvantage. With the technique above, I feel confident that I can now reduce the number of space leaks in my code.

Monday, September 14, 2015

Summary: We have a version of mapM for IO that takes O(1) stack and is faster than the standard Haskell/GHC one for long lists.

The standard Haskell/GHC base library sequence function in IO takes O(n) stack space. However, working with Tom Ellis, we came up with a version that takes O(1) stack space. Our version is slower at reasonable sizes, but faster at huge sizes (100,000+ elements). The standard definition of sequence (when specialised for both IO and []) is equivalent to:

First, we traverse the list using apply. Note that we pass the RealWorld token down the list (ensuring the items happen in the right order), but we do not pass it back.

To ensure all the IO actions performed during apply happen before we return any of the list, we then demand the list, ensuring the [] element has been forced.

Both these traversals use O(1) stack. The first runs the actions and constructs the list. The second ensures evaluation has happened before we continue. The trick in this algorithm is:

ys <- IO $ \r -> (# r, apply r xs #)

Here we cheat by duplicating r, which is potentially unsafe. This line does not evaluate apply, merely returns a thunk which when evaluated will force apply, and cause the IO to happen during evaluation, at somewhat unspecified times. To regain well-defined evaluation order we force the result of apply on the next line using demand.

Benchmarks

We benchmarked using GHC 7.10.2, comparing the default sequence (which has identical performance to the specialised monomorphic variant at the top of this post), and our sequenceIO. We benchmarked at different lengths of lists. Our sequenceIO is twice as slow at short lists, draws even around 10,000-100,000 elements, and goes 40% faster by 1,000,000 elements.

Our algorithm saves allocating stack, at the cost of iterating through the list twice. It is likely that by tuning the stack allocation flags the standard algorithm would be faster everywhere.

Using sequence at large sizes

Despite improved performance at large size, we would not encourage people to use sequence or mapM at such large sizes - these functions still require O(n) memory. Instead:

If you are iterating over the elements, consider fusing this stage with subsequence stages, so that each element is processed one-by-one. The conduit and pipes libraries both help with that approach.

If you are reducing the elements, e.g. performing a sum, consider fusing the mapM and sum using something like foldM.

For common operations, such a concat after a mapM, an obvious definition of concatMapM is: